Air classifier systems are commonly used to produce serviceable materials for use as fuels or to recover valuable resources from municipal refuse and the like. It has been found that an inclined rotary drum coupled with a plenum will efficiently separate light and heavy items from a supply of commingled materials if the drum is inclined at an optimum angle and if air through the system flows at an optimum rate of speed, and if certain other parameters are optimized.
Gibbons and Passanti in their U.S. Pat. No. 3,804,249 teach the use of a large rotating drum with its axis inclined to the horizontal with a blower for creating flow of air through the drum and upwardly through a plenum at the upper end of the drum. Commingled materials are introduced into the rotating drum and are tumbled continuously by the drum. Heavy items gravitate toward and out the lower end of the drum while light items are propelled by the air stream out the upper end of the drum into the plenum. Within the plenum the light materials drop to the bottom while the air is exhausted above.
Since the Gibbons et al development, however, air classifier systems have become much more exotic and include various components and modules which are intended to improve aspects of the classification and dust removal operations. One such improved system is disclosed in copending U.S. patent application Ser. No. 906,726, (now abandoned) filed May 17, 1978 by Malcolm M. Paterson and assigned to the same assignee as the present invention. However, practically no means has been found to satisfactorily increase the capacity of air classification system of this type or to classify simultaneously two materials having different densities or similar different characteristics. Prior to this invention it has been necessary to first classify one supply of material and then later to classify the second material, or to provide two separate complete systems, one for each material.